The explanation is from
http://people.debian.org/~stapelberg/docs/systemd-dependencies.html
---
man/systemd-halt.service.xml | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/man/systemd-halt.service.xml b/man/systemd-halt.service.xml
index 2fd7b8b..0737d05 100644
--- a/man/systemd-halt.service.xml
+++ b/man/systemd-halt.service.xml
@@ -83,6 +83,12 @@
remaining swap devices, detach all remaining storage
devices and kill all remaining processes.</para>
+ <para>It is necessary to have this code in a separate binary
+ because otherwise rebooting after an upgrade might be broken —
+ the running PID 1 could still depend on libraries which are not
+ available any more, thus keeping the filesystem busy, which
+ then cannot be re-mounted read-only.</para>
+
<para>Immediately before executing the actual system
halt/poweroff/reboot/kexec
<filename>systemd-shutdown</filename> will run all
--
1.8.2
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